In an audio and/or video disc information reading and reproducing apparatus such as an audio and/or video disc player presently in use, audio and/or video information is stored on an audio and/or video disc in the form of frequency-modulated audio and/or video carriers superposed on one another or in the form of pulse code modulated signals as in the case of an audio disc. Such multiplex or pulse-coded information is recorded as a series of depressed areas or "pits" formed in an information-carrying face of the audio and/or video and arranged in a spiral recorded track or in a number of concentric recorded tracks about the center axis of the disc. In an information reading and reproducing apparatus of the optically-scanned type, the audio and/or video information thus stored on the audio and/or video disc is read out by scanning the pits along a track on the disc with a beam of laser light and thereby detecting the lengths of and the spacings between the scanned pits. During playback of such an audio and/or video disc, the disc is driven for rotation about the center axis thereof usually at a fixed velocity with a beam of laser light emitted from an optical signal pick-up unit and focused on the target track on the rotating disc by means of tracking and focusing servos included in the pick-up unit. The beam of the light thus incident on the information-carrying face of the rotating audio and/or video disc is reflected therefrom or transmitted therethrough and is modulated by the audio and/or video signals represented by the pattern of the pits in the recorded track traced by the beam. The signals thus read out from the recorded track by the information pick-up beam are extracted from the beam and are converted into electric audio and/or video reproducing signals by means of a photoelectric transducer unit.
To enable the information pick-up beam to reach or search for a desired track of the disc or any predetermined position with respect to the disc being played back in an audio and/or video disc information reading and reproducing apparatus of the above described nature, the signal pick-up unit incorporated in the reading and reproducing apparatus is usually arranged to be movable radially of the audio and/or video disc. The signal pick-up unit of the information reading and reproducing apparatus is for this purpose mounted on a carriage provided with suitable drive means such as, typically, a reversible geared motor and suitable control means for controlling the velocity at which the pick-up unit is to be driven to move on the carriage by such drive means. The velocity control means is provided in the signal pick-up unit and is adapted to detect the output speed of the drive means and to produce an output signal in the form of a voltage variable with the detected output speed. The signal is compared with a reference signal represented by a predetermined voltage so as to produce an error signal corresponding to the difference between the voltages respectively representative of the signals compared with each other. A control signal is produced on the basis of this error signal and is fed back to the drive means for thereby controlling the output speed of the drive means.
Thus, the known velocity control means for the signal pick-up unit of a audio and/or video disc information reading and reproducing apparatus has included therein a velocity detecting device such as, for example, a tachometric generator or a frequency generator to detect the output speed of the drive means for the pick-up unit. Provision of such a detecting device in the signal pick-up unit not only results in an increased production cost of the information reading and reproducing apparatus as a whole but makes it difficult to construct the pick-up unit compactly with use of, for example, a semiconductor laser emitter in the pick-up unit.
It is, accordingly, a prime object of the present invention to provide an audio and/or video disc information reading and reproducing apparatus having an improved velocity control device having compact and small-sized construction and yet capable of reliably controlling the output speed of the drive means for the signal pick-up unit of the reading and reproducing apparatus.
It is another object of the present invention to provide an audio and/or video disc information reading and reproducing apparatus featuring a velocity control device capable of detecting the output speed of the drive means for the signal pick-up unit of the reading and reproducing apparatus without using a tachometric generator, a frequency generator or any other form of such a detecting device having unwieldy construction which may give rise to an increase in the production cost of the apparatus as a whole.